Aryl-capped iron-chelating siderophores assist various pathogens in acquiring iron inside their mammalian host, where iron is tightly chelated. The siderophores are essential for infection. In particular, siderophores are essential for infection by Mycobacterium tuberculosis, the causative agent for tuberculosis (de Voss et al. Proc. Natl., Acad. Sci. USA 97:1252-57, 2000; incorporated herein by reference), and Yersinia pestis, the etiological agent of the plague (de Almeid et al. Microb. Pathog. 14:9-21, 1993; Bearden et al. Infect. Immun. 65:1659-1668, 1997; each of which is incorporated herein by reference). Other pathogens which depend on siderophore-based iron acquisition systems include Yersinia enterolitica, Pseudomonas aeruginosa, Bacillus anthracis, Vibrio vulnificus, Yersinia ruckeri, Brucella abortus, Burkholderia cepacia, Burkholderia cenocepacia, Bordetella bronchiseptica, Acinebacter calcoaceticus, Escherichia coli, Salmonella enterica, Shigella spp., and Vibrio cholerae (Litwin et al., Infect. Immun. 64:2834-38, 1996; Bellaire et al., Infect. Immun. 71:1794-803, 2003; Boschiroli et al., Curr. Opin. Microbiol. 4:58-64, 2001; Sokol et al., Infect. Immun. 67:4443-55, 1999; Register et al., Infect. Immun. 69:2137-43, 2001; each of which is incorporated herein by reference). Inside their hosts, iron is relatively abundant but is tightly bound to intracellular and extracellular components (Weinberg, Perspect. Biol. Med. 36:215-221, 1993; incorporated herein by reference). The pathogenic bacteria synthesize siderophores to acquire Fe(III) from their hosts (Wooldridge and Williams, FEMS Microbiol. Rev. 12:325-348, 1993; incorporated herein by reference). Siderophore biosynthesis is, therefore, an attractive target for the development of new antibiotics to treat tuberculosis, plague, and other infection caused by microorganisms that depend on siderophore (e.g. Pseudomonas aeruginosa).
The two siderophore families produced by M. tuberculosis, the cell-associated and soluble mycobactins (MBTs) (Quadri et al. in Tuberculosis and Tubercle Bacillus (eds. Cole et al.) 341-57 (ASM Press, Washington, D.C., 2004); incorporated herein by reference), and the Y. pestis siderophore, yersiniabactin (YBT) (Perry et al. Microbiology 145:1181-90, 1999; incorporated herein by reference), have salicyl-capped non-ribosomal peptide-polyketide hybrid scaffold (FIG. 1A). Yersinia enterocolitica, Pseudomonas aeruginosa, Acinebacter calcoaceticus, and A. baumannii also produce phenolic siderophores (also known as “salicyl-capped siderophores”). Other pathogens such as E. coli, Salmonella enterica, Shigella spp., and Vibrio cholerae produce closely related catechol-containing siderophores such as vibriobactin, anguibactin, and enterobactin (FIG. 1A). Siderophore biosynthetic pathways have undergone extensive investigations (Quadri, Mol. Microbiol. 37:1-12, 2000; Crosa et al. Microbiol. Mol. Biol. Rev. 66:223-49, 2002; each of which is incorporated herein by reference). During the biosyntheses of MBTs, YBT, and other phenolic siderophores, domain salicylation enzymes, such as MbtA and YbtE respectively, catalyze the salicylation of an aroyl carrier protein (ArCP) domain to form a salicyl-ArCP domain thioester intermediate via a two-step reaction (FIG. 1B) (Quadri et al. Chem. Biol. 5:631-45, 1998; Gehring et al. Biochemistry 37:11637-11650, 1998; each of which is incorporated herein by reference). The first step is ATP-dependent adenylation of salicylate to generate a salicyl-AMP intermediate (FIG. 1C), which remains non-covalently bound to the active site. The second step is the transesterification of the salicyl moiety onto the thiol of the phosphopantetheinyl prosthetic group of the ArCP domain (Quadri et al. Chem. Biol. 5:631-45, 1998; Gehring et al. Biochemistry 37:11637-11650, 1998; each of which is incorporated herein by reference). Since MbtA and YbtE have no homologs in humans, they are particularly attractive targets for the development of novel antibiotics that inhibit siderophore biosynthesis. Related 2,3-dihydroxybenzoate adenylation enzymes are involved in the biosynthesis of catechol-containing siderophores (also known as “2,3-dihydroxybenzoate-capped siderophores”). Other mechanistically related adenylate-forming enzymes have been shown to bind their cognate acyl-AMP intermediates 2-3 orders of magnitude more tightly than their carboxylic acid and ATP substrates (Kim et al. Appl. Microbiol. Biotechnol. 61:278-88, 2003; incorporated herein by reference). Among these are the acyl sulfamoyl adenosines (acyl-AMS) (Kim et al. Appl. Microbiol. Biotechnol. 61:278-88, 2003; Finking et al. ChemBioChem 4:903-906, 2003; each of which is incorporated herein by reference), inspired by the natural products, nucleodin (4), ascamycin (5), and AT-265 (6) (FIG. 1C).
Mechanism-based inhibitors of salicylation enzymes could be used to treat infection such as tuberculosis and the plague by inhibiting the salicylate adenylation activity of YbtE and MbtA. These compounds may also be useful in treating other infections caused by organisms which rely of siderophore-based iron acquisition systems. Therefore, inhibitors of salicylate adenylation enzymes would provide a new mechanism of action in combating infections, particularly ones caused by drug-resistant organisms.